Propanol is a solvent used industrially, but more importantly, it can be readily dehydrated to produce propylene which is the second largest chemical commodity in the world with production of >70 million tons/per year. Currently propylene is produced mainly by steam-cracking of naphtha or liquid petroleum gas or fluid catalytic cracking of gasoils in very large installations as a secondary product. The steam-cracking a process that makes mainly ethylene and many other co-products, such as butylenes, butadiene and pyrolysis gasoline, all of which need to be purified and to be utilized simultaneously. Other ways to make propylene is in a refinery FCC (fluid catalytic cracking) where propylene is a byproduct from heavy gasoil cracking in proportions between 3 and 15 wt %. Propylene can also be produced by catalytic dehydrogenation of propane. Still another way to make propylene is via metathesis of butenes with ethylene.
Since for many centuries, simple sugars are being fermented into ethanol with the help of saccharomyces cerevisae. The last decade's new routes starting from cellulose and hemicelluloses have been developed to ferment more complex carbohydrates into ethanol. Hereto, the carbohydrates need to be unlocked from the lignocellulosic biomass. Biomass consists approximately of 30% cellulose, 35% hemicelluloses and 25% lignin. The lignin fraction cannot be valorised as ethanol because of its aromatic nature, and can only be used as an energy source which is present in many cases in excess for running an industrial plant.
Several microorganisms are able to use one-carbon compounds as a carbon source and some even as an energy source. Carbon dioxide is an important carbon source for phototrophs, sulfate reducers, methanogens, acetogens and chemolithotrophic microorganisms. There are essentially four systems to fix CO2: (1) the Calvin cycle [CO2 fixing enzyme: ribulose-1,5-bisphosphate carboxylase], (2) the reductive citric acid cycle [CO2 fixing enzymes: 2-oxoglutarate synthase, isocitrate dehydrogenase, pyruvate synthase], (3) the acetyl-CoA pathway [CO2 fixing enzyme: acetyl-CoA synthase, linked to CO-dehydrogenase] and (4) the 3-hydroxypropionate cycle [CO2 fixing enzyme: acetyl-CoA carboxylase, propionyl-CoA carboxylase] (“Structural and functional relationships in Prokaryotes”, L. Barton, Springer 2005; “Carbon monoxide-dependent energy metabolism in anaerobic bacteria and archaea”, E. Oelgeschelager, M. Rother, Arch. Microbiol., 190, p. 257, 2008; “Life with carbon monoxide”, S. Ragsdale, Critical Reviews in Biochem. and Mol. Biology, 39, p. 165, 2004).
More recently, more efficient routes that produce synthesis gas from carbon-containing materials and that subsequently is fermented into ethanol are being developed (“Bioconversion of synthesis gas into liquid or gaseous fuels”, K. Klasson, M. Ackerson, E. Clausen, J. Gaddy, Enzyme and Microbial Technology, 14(8), p. 602, 1992; “Fermentation of Biomass-Generated Producer Gas to Ethanol”, R. Datar, R. Shenkman, B. Cateni, R. Huhnke, R. Lewis, Biotechnology and Bioengineering, 86 (5), p. 587, 2004; “Microbiology of synthesis gas fermentation for biofuel production”, A. Hemstra, J. Sipma, A. Rinzema, A. Stams, Current Opinion in Biotechnology, 18, p. 200, 2007; “Old Acetogens, New Light”, H. Drake, A. Göβner, S. Daniel, Ann. N.Y. Acad. Sci. 1125: 100-128, 2008).
Synthesis gas can be produced by gasification of the whole biomass without need to unlock certain fractions. Synthesis gas can also be produced from other feedstock via gasification: (i) coal, (ii) municipal waste (iii) plastic waste, (iv) petcoke and (v) liquid residues from refineries or from the paper industry (black liquor). Synthesis gas can also be produced from natural gas via steam reforming or autothermal reforming (partial oxidation).
The biochemical pathway of synthesis gas conversion is described by the Wood-Ljungdahl Pathway. Fermentation of syngas offers several advantages such as high specificity of biocatalysts, lower energy costs (because of low pressure and low temperature bioconversion conditions), greater resistance to biocatalyst poisoning and nearly no constraint for a preset H2 to CO ratio (“Reactor design issues for synthesis-gas fermentations” M. Bredwell, P. Srivastava, R. Worden, Biotechnology Progress 15, 834-844, 1999; “Biological conversion of synthesis gas into fuels”, K. Klasson, C. Ackerson, E. Clausen, J. Gaddy, International Journal of Hydrogen Energy 17, p. 281, 1992). Acetogens are a group of anaerobic bacteria able to convert syngas components, like CO, CO2 and H2 to acetate via the reductive acetyl-CoA or the Wood-Ljungdahl pathway.
Several anaerobic bacteria have been isolated that have the ability to ferment syngas to ethanol, acetic acid and other useful end products. Clostridium ljungdahlii and Clostridium autoethanogenum, were two of the first known organisms to convert CO, CO2 and H2 to ethanol and acetic acid. Commonly known as homoacetogens, these microorganisms have the ability to reduce CO2 to acetate in order to produce required energy and to produce cell mass. The overall stoichiometry for the synthesis of ethanol using three different combinations of syngas components is as follows (J. Vega, S. Prieto, B. Elmore, E. Clausen, J. Gaddy, “The Biological Production of Ethanol from Synthesis Gas”, Applied Biochemistry and Biotechnology, 20-1, p. 781, 1989):6CO+3H2O→CH3CH2OH+4CO2 2CO2+6H2→CH3CH2OH+3H2O6CO+6H2→2CH3CH2OH+2CO2 The primary product produced by the fermentation of CO and/or H2 and CO2 by homoacetogens is ethanol principally according to the first two of the previously given reactions. Homoacetogens may also produce acetate. Acetate production occurs via the following reactions:4CO+2H2O→CH3COOH+2CO2 4H2+2CO2→CH3COOH+2H2O
Clostridium ljungdahlii, one of the first autotrophic microorganisms known to ferment synthesis gas to ethanol was isolated in 1987, as an homoacetogen it favors the production of acetate during its active growth phase (acetogenesis) while ethanol is produced primarily as a non-growth-related product (solventogenesis) (“Biological conversion of synthesis gas into fuels”, K. Klasson, C. Ackerson, E. Clausen, J. Gaddy, International Journal of Hydrogen Energy 17, p. 281, 1992).
Clostridium autoethanogenum is a strictly anaerobic, gram-positive, spore-forming, rod-like, motile bacterium which metabolizes CO to form ethanol, acetate and CO2 as end products, beside it ability to use CO2 and H2, pyruvate, xylose, arabinose, fructose, rhamnose and L-glutamate as substrates (J. Abrini, H. Naveau, E. Nyns, “Clostridium autoethanogenum, Sp-Nov, an Anaerobic Bacterium That Produces Ethanol from Carbon-Monoxide”, Archives of Microbiology, 161(4), p. 345, 1994).
Anaerobic acetogenic microorganisms offer a viable route to convert waste gases, such as syngas, to useful products, such as ethanol, via a fermentation process. Such bacteria catalyze the conversion of H2 and CO2 and/or CO to acids and/or alcohols with higher specificity, higher yields and lower energy costs than can be attained by traditional production processes. While many of the anaerobic microorganisms utilized in the fermentation of ethanol also produce a small amount of propanol as a by-product, to date, no single anaerobic microorganism has been described that can utilize the fermentation process to produce high yields of propanol.
Therefore a need in the art remains for methods using microorganisms in the production of propanol using fermentation with a substrate of H2 and CO2 and/or CO.